In particular in power tools comprising a reciprocatingly driven tool bit the problem arises that vibrations generated by the drive mechanism for the tool bit are transferred to the user who is operating the tool. Since operating a vibrating power tool is considered uncomfortable and may have negative effects on the health of the user, there is a growing need to reduce the vibrations applied to a user during use of such power tool.
In a powered hammer the hammer mechanism usually comprises a hollow spindle or cylinder in which a ram is slidably arranged and a tool holder disposed at the front end of the spindle for supporting a tool bit, the bit being capable of sliding to a limited extend along an axis being parallel to the spindle axis. Further, a piston is guided within the spindle or cylinder wherein an air cushion is provided between the piston and the ram. The piston is coupled to a crank drive so that a rotational movement of a drive motor shaft of the hammer is converted into a reciprocating movement of the piston. This movement in turn is transferred to the ram via the air cushion, the ram hitting either directly a tool bit supported by the tool holder or a beat piece arranged between the ram and the tool bit wherein in both cases the momentum of the ram is transferred to the tool bit.
During normal use of a powered hammer, when the drive motor is activated and the ram applies impacts on the tool bit, vibrations of the entire hammer are generated wherein these vibrations are felt by the user carrying the hammer. If the amplitude of these vibrations exceeds certain thresholds, this may cause serious damages to the user's health in case the hammer is used over a sufficiently long period. In particular, problems may occur in the region of the user's hands, arms and shoulders.
As a result the legal stipulations regarding vibrations of tools to which employees are subjected, have recently been tightened. In particular, the threshold values for vibrations above which the health conditions of an employee have to be monitored in case the employee is subjected to these vibrations have been reduced significantly. Therefore, it is required that power tools are adapted to comply with these new rules in order to avoid additional efforts for the employer. In particular, the amplitude of the vibrations occurring at the handle portions should be minimized.
To this end as a counter measure against vibrations, it is known from the prior art to employ an oscillating counter mass in the hammer. Here, EP 1 252 976 A1 discloses to provide a slidable counter mass in the tool housing, the mass being supported by a spring assembly and being slidable along a direction which is parallel to the moving direction of the ram. This spring-mass-assembly has a resonance frequency which is mainly determined by the spring stiffness, the weight of the counter mass and the dampening effect due to friction.
Due to the vibrations generated by the hammer mechanism, oscillations of the mass are induced wherein these vibrations have a frequency which is equal to the frequency with which the ram applies impacts on the beat piece and the tool bit, respectively. Thus, the vibration frequency is determined by the rotational speed of the drive motor.
If the vibration frequency, i.e. the frequency with which the spring-mass-assembly is excited, is below the resonance frequency of the spring-mass-assembly, the mass oscillates in anti-phase with the ram. This leads to a reduction of the overall vibrations of the tool housing wherein the system is most efficient if the vibration frequency is close to but below the resonance frequency, since then the amplitude with which the counter mass oscillates is maximized.
However, here the following problem occurs. If the vibration frequency exceeds the resonance frequency of the spring-mass-assembly, the mass oscillates in parallel with the ram rather than being in anti-phase, which has the negative effect that the vibrations of the entire tool are enhanced rather than being reduced.
Therefore, it has to be ensured that the resonance frequency of the mass spring system is above the vibration frequency. In this connection, tolerances have to be taken into account that occur during production of the springs of the spring-mass-assembly.
In order to ensure that the aforementioned requirement for the resonance frequency is fulfilled independent of the tolerances of the springs, the design of the spring-mass-assembly is chosen such that the calculated value of the resonance frequency of the system is well above the vibration frequency which is determined by the rotational speed of the electric motor. However, this results in a vibration dampening effect which is less compared to the case in which the vibration frequency nearly reaches the resonance frequency and the oscillation amplitude of the counter mass reaches a maximum value at which the windings of the springs do not get into contact with each other.